This invention relates to an in-mold method and apparatus for sulfonating plastic articles during molding and to the sulfonated plastic articles so produced. More particularly, it relates to a system for injecting sulfur trioxide into a pre-mold chamber, forming a relatively high-pressure gaseous sulfur trioxide, and then, introducing the gaseous sulfur trioxide into the mold cavity during molding of plastic articles such as plastic containers.
The use of resinous organic polymers to fabricate enclosure members (plastic containers) such as bottles, tanks and other containers and other molded plastic articles is well known. Plastic containers which are made from most organic polymers, particularly the dominantly hydrocarbon polymers, are non-electroconductive, are non-adherent to polar materials such as inks, paints, dyes and various polar adhesives and are readily permeated and/or stress cracked or swollen by oleophilic materials such as liquid and gaseous hydrocarbons e.g., solvents, fumes, vapors, benzene, cyclohexane, xylene, chlorinated solvents, and hexane; gases such as nitrogen, oxygen, helium, hydrogen, carbon dioxide, methane, ethane, propane, butane, halocarbons; fuels such as gasoline, kerosene, fuel oils; oils such as natural fatty oils, lubricating oils, perfumes and agricultural chemicals. Depending on the particular polymer, these materials can adversely affect the container material. For example, natural fatty oils tend to cause stress cracking of polymers formed from olefinic monomers such as polyethylene. As a result of these inherent deficiencies many of such organic polymers must be treated with various agents which impart varying degrees of electroconductivity, adhesion and impermeability.
Sulfonation techniques have been developed as one means of treating plastic articles to decrease the permeability and protect the polymeric material. See, for example, Walles U.S. Pat. Nos. 2,832,696; 2,937,066; 3,592,724; 3,613,957; 3,625,751; 3,629,025; 3,740,258; 3,770,706; 3,959,561; 4,220,739 and 4,615,914. Of these, U.S. Pat. Nos. 3,613,957, 3,740,258 and 4,615,914 are particularly directed to plastic containers for organic fluids such as vehicle gasoline tanks and polyethylene drums for transport and storage of industrial solvents and the like.
Walles U.S. Pat. Nos. 3,613,957 and 3,740,258 relate to a post-mold treatment of plastic containers fabricated of non-aromatic hydrocarbon polymers. After the plastic containers have been extruded or molded, the surface to be sulfonated, preferably the interior surface of the plastic container, is exposed to gaseous sulfur trioxide, preferably diluted with a dry inert gas such as dry air. Alternatively, the interior surface may be contacted with a solution of sulfur trioxide in an inert liquid solvent, such as a liquid chlorofluorocarbon.
The interior surface sulfonated plastic containers are then substantially completely neutralized by contacting them with a dilute aqueous solution of alkali metal hydroxide, ammonium hydroxide, gaseous ammonia or other neutralizing agents. During the neutralization step, salts such as ammonium sulfate and ammonium sulfamate (when ammonia is used as the neutralizing agent) are formed. While such salts are rinseable and/or leachable, washing, rinsing and drying are required to remove these salts. This is especially necessary when the plastic container is to be used as a vehicle gasoline tank because residual salts are not soluble in gasoline and can clog the fuel line and/or fuel filter stalling or otherwise hindering the operation of the vehicle.
The washing, rinsing and drying steps are time consuming under any circumstances and are particularly so when the plastic container has a complicated shape which makes it difficult to thoroughly rinse and drain. With vehicle gasoline tanks, for example, it is often necessary to rinse completely and, then suspend the tank upside down for drainage and drying. The effort and time involved are considerable.
In Walles U.S. Pat. No. 4,615,914 the interior surfaces of plastic containers are post-mold treated by conversion of solid pills of polymeric sulfur trioxide into an air/sulfur trioxide gas mixture via microwave energy. This sulfonation process leaves no residue, but still upon neutralization with ammonia gas (which may be formed from solid ammonium carbonate), salts such as ammonium sulfamate and ammonium carbamate are produced. Accordingly, in Walles U.S. Pat. No. 4,615,914 rinsing is again suggested in order to remove such secondary reaction products.
The need which exists for a treatment process which alleviates the requirement for washing, rinsing and drying, has been met somewhat by use of an in-mold fluorination treatment. Dixon et al in U.S. Pat. No. 3,862,284 teach that in the blow molding of thermoplastic resins to produce plastic containers, 0.1-20% by volume fluorine and 99.9-80% by volume of an inert gas are blended into a fluid medium before expanding the parison of the plastic container to the contour of the mold. See also U.S. Pat. Nos. 4,336,015 and 4,396,567 to Rainville. Plastic containers produced by this process do not require washing, rinsing and drying. However, handling the fluorinating chemicals presents problems because of their corrosive and toxic nature. In addition the fluorinated surface does not provide as good a barrier layer as a sulfonated one.
An in-mold sulfonation system would, therefore, be desirable in that it could combine the best features of both post-mold sulfonation and in-mold fluorination and thereby eliminate many of the disadvantages of each. But, previously it has not been possible to satisfactorily conduct an in-mold sulfonation process. To a large extent this has been because it is difficult to inject gaseous sulfur trioxide into a mold which is under relatively high pressure and to control the time and temperature of the sulfonation treatment so as to achieve the deserved result.
Accordingly, the need exists for an effective and efficient in-mold sulfonation method and apparatus for sulfonating plastic articles during molding.